Electric power steering apparatuses are provided for driving a motor according to information of a steering torque signal, a vehicle speed, etc., so as to achieve a function of reducing a steering force of a driver. If a failure (abnormality) occurs in a main-CPU (Central Processing Unit) for controlling the motor, it is necessary to restrict the output of the motor in order to ensure the safety. Meanwhile, as a method for restricting the motor output irrespective of the presence or absence of a failure of the main-CPU, there is a method using so-called interlock means, in which a region for prohibiting the output is determined with respect to a relationship between the (steering) torque signal and a motor-drive current signal, so that the motor output is prohibited when it falls in this output prohibited region. Instead, there is a method in which a sub-CPU for monitoring the motor-controlling main-CPU is provided, so that the energization of the motor is suspended when the sub-CPU detects a failure of the main-CPU.
Further, since suspending the driving of the motor makes it difficult to rotate the steering wheel, resulting in a possibility that it becomes unable to cause the running itself of the vehicle, there is also a case where the controlling is continued as long as possible depending on the content of the failure. This is exemplified by the case where the controlling is continued using a sub-torque signal when a main-torque signal is abnormal.